1. Field of the Invention
In the pulverizing carbonaceous material e.g. coal, as a feedstock, particularly as a fuel for burners, a large amount of small particulate coal is produced, which with air can provide an explosive mixture. During operation of the mill, the large amount of fuel which is present, appears to inhibit explosions. However, during start-up and shutdown, the coal-air mixture must pass through a composition zone of explosive mixtures in going from the air rich to the fuel rich composition.
The problem is exacerbated with fuels, such as sub-bituminous coal. In the operation of the mill, air is employed at elevated temperatures. The sub-bituminous coal can become sticky and agglomerate and adhere to the hot walls of the mill. The coal can then begin to burn and/or remain in the mill during shutdown and subsequent start-up, which can act as an ignition source when the fuel-air composition passes through the explosion zone.
In attempting to inhibit explosions during start-up and shutdown, there are many considerations. The large volume of the mill inhibits the use of an expensive material, since large volumes of the material will be required. Furthermore, one cannot use some materials, because of their toxic or adverse physiological effects. In addition, one must be concerned about the effect of any inerting material on the coal, where the nature of the material is to leave a residue, particularly, as to the thermal efficiency of the coal. It is found that pulverized bituminous coal, and sub-bituminous even more so, will agglomerate in the presence of water and strongly adhere to the walls, requiring mechanical cleanout for safe operation. Therefore, in the past, when steam has been used for inerting the pulverizing system, it has been employed after the pulverizer has been shutdown and is in the sealed or bottled up condition. The system is then swept with air, to insure the substantial removal of any moisture, before the introduction of coal into the mill.
2. Brief Description of the Prior Art
U.S. Pat. No. 2,565,420 teaches the continued addition of water during grinding of various organic materials, where the heat of grinding vaporizes the water to provide an inert atmosphere. Descriptions of furnace explosion problems may be found in Coykendall, "Furnace-Boiler Fuel Explosion Protection," ASME Paper 64-PWR-8, Livingston, "Preventing Furnace Explosions Part No. 2," Combustion Engineering, and Fisher, "Development and Operating Experience with Inerting Systems on Coal Pulverizers and Bunkers," American Power Conference, 1978.